feat: add progress tacker logger to pregel context

[frank-zsy]

Signed-off-by: Frankzhaopku syzhao1988@126.com

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This PR tends to add a build-in logger to Pregel framework. close #180

I looked into the code and InitContext and ComputeContext derive from NodeCentricContext which derives from PregelContext. And MasterComputeContext derives from PregelContext, so I add a private ProgressTracker reference into the PregelContext and expose three public functions logDebug, logWarning and logMessage which will call the same functions in the tracker, so developers can use them in all three contexts in the framework.

What I am not sure is:

• How to add or should I add test case to this PR?

[frank-zsy]

@s1ck could you have a look on this pull please?

[s1ck]

@frank-zsy Thanks for the PR. I was afk last week and didn't see it until now! Will have a look.

[frank-zsy]

Thanks, should I add the Logging section to 2.4 just before Message reducer or 2.6?

[s1ck]

Thanks, should I add the Logging section to 2.4 just before Message reducer or 2.6?

Let's do 2.6 Logging and briefly describe that the "following methods" are available at the three context objects and that they allow us to inject custom messages into the progress log of the algorithm execution.

[frank-zsy]

Thanks for the review, I looked into the documentation carefully and found almost equal number of function and method, is there any convention when to use which?

[s1ck]

Thanks for the review, I looked into the documentation carefully and found almost equal number of function and method, is there any convention when to use which?

It's true that it's mixed up, but since we use Java code snippets here, using method seems appropriate.

[frank-zsy]

Sure, thanks.